[Linux-cluster] Mirroring GFS

[Benjamin Marzinski]

On Thu, Apr 28, 2005 at 04:42:44PM +0200, Andrea Manca wrote:
> Hi to all.
> 
> I've a couple of question about interaction between MD devices (software
> RAID), GNBD, and GFS.
> 
> I'm testing a cluster with four nodes: two gfs and two gnbd nodes.
> GNBD1 and GNBD2 export each one a partition over the LAN;
> GFS1 and GFS2 import that exported block devices.
> 
> I'm interested in mirroring the information in GNBD1 over GNBD2, so i've
> thought about creating a md device composed by
> 
> /dev/gnbd/GNBD1_PRIMARY
> /dev/gnbd/GNBD2_MIRROR
> 
> in RAID1 configuration and gfs formatted.
> It seem's a reasonable configuration for me, but in:
> 
> https://www.redhat.com/archives/linux-cluster/2004-October/msg00055.html
> 
> a similar solution (except for a RAID5 configuration), is discussed and
> marked as impossible, because Software Raid in linux isn't cluster
> aware. This could lead an inconsistency in parity blocks which aren't
> locked correctly.
> In my opinion two mirrored disks doesn't need a block level locking,
> because parity block aren't present.
> 
> i've tried this on 4 VMWare machines and it seem work well if you write
> (or read) something from GFSx nodes.
> 
> Could my solution get working or there is a similar issue applicable in
> my situation?
> In this second case, are any workarounds possible?

Unfortunately, this won't work.  In your situation, say that GFS1 dies while
in the middle of a write, so that the write completes to GNBD1, but not GNBD2.
The mirror would be out of sync, but there would be no way for GFS2 to know
that.  There is cluster aware mirroring code under development that will fix
this problem.

Sorry
-Ben
 
> I really appreciate any comment or explanation.
> 
> Thank you all in advance.
> 
> 
> Andrea.
> manca at link.it
> 
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